1. Field of the Invention
The present invention relates to a golf ball and more particularly to a dimple pattern on a golf ball.
2. Description of the Related Art
A golf ball has approximately 300 to 550 dimples on its surface. The role of the dimples resides in one aspect that such dimples disturb the air stream around the golf ball during its flight to accelerate the transition of turbulent flow at a boundary layer of the golf ball, thereby causing a turbulent flow separation (which will be hereinafter referred to as the xe2x80x9cdimple effectxe2x80x9d). The acceleration of the transition of the turbulent flow causes a separating point of air from the golf ball to be shifted backward so that the pressure resistance is reduced. Moreover, the acceleration of the transition of the turbulent flow increases a differential between upper and lower separating points of the golf ball which is caused by back spin. Consequently, the lift acting on the golf ball is increased. By a reduction in the pressure resistance and an enhancement in the lift, the flight distance of the golf ball is increased. A dimple pattern capable of readily promoting the transition of the turbulent flow, that is, a dimple pattern capable of effectively disturbing the air stream is more aerodynamically excellent.
There have been proposed various golf balls having improved dimple patterns in order to enhance a flight performance. For example, Japanese Patent Publication No. Sho 58-50744 (U.S. Pat. No. 5,080,367) has disclosed a golf ball in which dimples are densely provided such that a pitch between the dimples is 1.62 mm or less if possible. Moreover, Japanese Laid-Open Patent Publication No. Sho 62-192181 (U.S. Pat. No. 4,813,677) has disclosed a golf ball in which dimples are densely provided so as not to form a new dimple having an area which is equal to or larger than a mean area in a land portion other than the dimples. Furthermore, Japanese Laid-Open Patent Publication No. Hei 4-347177 (U.S. Pat. No. 5,292,132) has disclosed a golf ball in which dimples are provided very densely such that the number of land portions in which a rectangle having a predetermined dimension can be drawn is 40 or less.
All the golf balls disclosed in the known publications have densely provided dimples, in other words, the surface area occupation ratio of the dimple is increased. Those skilled in the art have recognized that the surface area occupation ratio is one of important elements for influencing the dimple effect.
The most important performance required for the golf ball by a golfer is flight performance. A long flight distance gives the golfer a refreshing feeling, and furthermore, contributes to an enhancement in the score. While a golf ball having its surface area occupation ratio increased exhibits an excellent flight performance, the golfer desires a further enhancement in the flight distance.
The present invention provides a golf ball having a large number of dimples on a surface thereof. In the golf ball, when a plurality of crown portions distributed over a phantom spherical surface of the golf ball are assumed to have a total area which is one-half of the area of the phantom spherical surface, the difference between the dimple occupation ratio Yc (%) in the crown portions and a dimple occupation ratio Yn (%) in a non-crown portion is 5% to 30%.
In the golf ball, the difference between the dimple occupation ratio Yc (%) and the dimple occupation ratio Yn (%) is greater than that of an ordinary golf ball. In other words, the dimples are dense in one of the crown portions and in a non-crown portion and are sparse in others. In the golf ball, the region in which the dimples are dense and the region in which the dimples are sparse appear alternately by backspin during flight. Consequently, the dimple effect of disturbing an air stream is promoted so that the flight distance of the golf ball can be increased.
Preferably, the areas of all the crown portions are equal to each other and dimple patterns in all the crown portions are almost equivalent to each other. The golf ball is excellent in aerodynamic symmetry.
With respect to aerodynamic symmetry, it is preferable that the crown portions be distributed as uniformly as possible. More specifically, it is preferable to obtain a golf ball in which all the crown portions are present in positions corresponding to the vertexes of a regular polyhedron inscribed in the phantom spherical surface. Moreover, the latitudes of all the crown portions excluding the crown portions positioned on the poles may be identical to each other.
It is preferable that a surface area occupation ratio Y is 70% to 90%. As described above, the dimples are comparatively sparse in one of the crown portions and in the non-crown portion. Also in this case, the surface area occupation ratio Y is set within the range. Consequently, the dimple effect of the whole golf ball can be prevented from being reduced. The expression xe2x80x9csurface area occupation ratio,xe2x80x9d as used in this specification, implies a ratio of the total dimple area to the area of the phantom spherical surface.